Image processing systems today process images in several stages. The most common way to process an image for later exposing it, is to save it in vector form, such as Gerber or Postscrit language, in a computer memory. This form permits one to save large images in a relatively small area of memory. In the process, the vector file is converted by a Raster Image Processor (RIP) into a bit map or run-length (RL) which represents the image. Each run-length is composed of two numbers, the first representing where a line starts, and the second, where the line ends. This is a convenient method for saving images on a medium sized memory. Usually the memory contains three dimensions, X, Y and Z, the Z dimension being a buffer, called the Z-buffer.
The RL is processed to an image by an apparatus which converts it to light to expose photosensitive material. In the process, a light modulator activates a laser light beam in accordance with the bit map to illuminate or not illuminate each pixel, one after the other.
Currently, there are three main methods for exposing and printing a saved image which are known in the art:
1) An internal drum system (such as is described in U.S. Pat. No. 4,853,709 to Stein et al) which is based on a stationary cylindrical surface to whose internal side is attached photosensitive material, and which uses a rotating prism to scan an image in order to recreate it on the photosensitve material;
2) An external drum system (as in U.S. Pat. No. 4,577,933 to Yip et al) in which the photosensitive material is attached by vacuum to the external side of a rotating drum, and in which a carriage moves along the drum axis and contains a writing head which creates the image from a modulator external to the drum; and
3) A flatbed system (as in U.S. Pat. No. 4,354,196 to Neumann et al) in which the photosensitive material is attached to a flat table bed which moves slowly along the X axis while a revolving multi - mirror polygon or prism scans the image in the Y axis.
In all of these systems the image is processed by modulating beams of light from a high brightness light source such as a laser diode or gas laser in order to expose the material pixel by pixel. The laser is modulated by means of a stationary light modulator which can be, for example an acousto - optical device such as is described in the aforementioned U.S. Pat. No. 4,577,933, a multi beam imager such as is described in U.S. Pat. No. 4,506,275 to Maeda or an integrated electronics device such as described in U.S. Pat. No. 4,367,925 to Sprague et al.
In all of the above modulators, the throughput is very low (several minutes for an 18".times.24" image) because of the limited number of actual writing beams. Furthermore, these systems cannot use large area light sources due to the low brightness obtained, which is insufficient for exposing individual pixels.
U.S. Pat. No. 5,049,901 to Gelbart describes a light modulator capable of using large area light sources, comprising a two dimensional deformable mirror modulator and moving photosensitive material. The modulator consists of a number of rows--preferably about 100--with about 1000 mirrors per row. The information to be imaged is entered into the first row and then transferred, row by row, to subsequent rows in a direction and rate such that the imaged data supposedly is kept stationary relative to the moving photosensitive material.
This patent suffers from several disadvantages:
1. The actual writing time for the whole image depends on the propagation delay time and the rise time of the electrical deformable mirror. For example: if the writing time for one row is 1 uSec, then for 100 rows the writing time will be 0.1 milli-second. The device has to rewrite all of the data for each row in turn. Therefore, the write time for an image containing 10,000,000,000 pixels (10.sup.10 which is a typical number of image-pixels in the art, for a 31".times.40" image at 8000 dpi) will be very long.
2. The deformable mirror device currently available is relatively small (100.times.1000 cells). Therefore, in order to increase the throughput, the system must contain several devices which will in all probability decrease the system's accuracy, repeatability and reliability.
3. The data moves sequentially from row to row in discrete quantities (i.e. from cell to cell), while the photosensitive material obviously moves continuously. This method causes the image's pixels to blur. The intensity pattern on the photosensititve material will be the convolution between the discrete data rate and the continuous velocity of the photosensitive material. As a result of the above, the actual pixel size will be almost double the original imaged pixel. The fact that the deformable mirror can change the data only one row at a time, increases that phenomenon.
Another disadvantage of all the above prior art systems is that the conversion from a vector file to an image is done by a separate computer. Therefore these systems are very expensive, require high-power computers and use highly complex algorithms. Moreover, all these systems use conventional memory to save the image in the computer (Static RAM or Dynamic RAM chips) and transfer the bits to an electronically controlled light modulator when exposing the photosensitive material. This results in a trade-off between the resolution and the imaging time (or throughput), because of the limited rate at which the data can be transferred in those devices.
There are also known rotating prism cameras, such as described in Soviet Patents numbers 1277055 and 1290240. In these cameras, the photosensitive material moves in a continuous manner while a rotating prism shifts the exposing image in synchrony with the photosensitive material. The prism does not modulate the image, but only transfers an image from a lens. There is no optical memory involved.